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CONWAY — A man who was injured when his house blew up Monday afternoon had apparently removed a propane insert from his fireplace earlier in the day, a state fire official said Tuesday.

Philip Murphy was seriously hurt in the propane explosion, which took place shortly after 4 p.m. at his home at 654 Main St.

A preliminary investigation shows he had removed the fireplace insert, said New Hampshire Fire Marshal Bill Degnan.

“Somewhere along the line, a leak was created in this house,” Degnan said. “Gas is a very efficient fuel, but when used improperly it’s very volatile.

Degnan said Murphy’s condition at Maine Medical Center in Portland was upgraded to fair as of Tuesday afternoon.

Fire crews arriving on the scene after being called at 4:10 p.m. found Murphy seriously injured lying in the grass. EMTs tended to him while fire crews took care of the smoke that was coming from the demolished home.

Fire Chief Stephen Solomon said that from the damage pattern, it is clear there was a propane explosion that originated in the basement.

Solomon said Murphy smelled gas when he opened the door shortly after 4 p.m. Monday. He went outside, turned off the gas and then went back into the home to see about the leak.

“He took four or five steps into the building and the building exploded,” Solomon said.

In 2007, New Hampshire legislators rewrote the law dealing with gaslines and required that licensed technicians install them. But Degnan said lawmakers allowed an exception for individual homeowners, an exception that the propane industry opposed.

Earlier Tuesday, Solomon said investigators did not know if a mechanical problem or a leaky valve caused the explosion.

Degnan said state Fire Marshal investigators have been at the scene all day Tuesday trying to decipher what happened.

Solomon said the second floor of the home collapsed onto the first floor. Only a couple of walls on the south end of the building, which dated to the mid- to late 1800s with several additions, remain standing. Debris was scattered for about 75 to 100 feet.

One of the first floor walls was blown off its foundation, and landed upside down up against a tree, he said.

There are three buildings on the property, which straddles the town line with Albany.

The other two buildings, an apartment house and the Bricks & Stones Masonry business, which the Murphy also owns, are located in Albany. They were not damaged.